fix: Add sampleCountMetricId to CustomAlarmThreshold (#581)

This provides support for `minSampleCountToEvaluateDatapoint` when using eg. `ErrorAlarmFactory`. Without this change and with a `MathExpression` containing multiple metrics, one would get the following error:

> "sampleCountMetricId must be specified when using minSampleCountToEvaluateDatapoint with a multiple-metric MathExpression"

However `sampleCountMetricId` is currently not available in `CustomAlarmThreshold`.

See #466 for original change.

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Author: bamarni